Stents can be used in a variety of tubular structures in the body including, but not limited to, ureters, common bile ducts, blood vessels, and the like. A stent may be used to expand a body tubular structure, maintain the lumen after expansion of the tubular structure or repair a damaged tubular segment. Stents are used, for example, after angioplasty and after atherectomy to maintain expanded lumen and to overlie an aortic dissecting aneurysm, and in a by-pass graft or a native vessel. Intraluminal endovascular prosthetic grafting is an alternative to conventional vascular surgery. Intraluminal endovascular grafting involves the percutaneous insertion into a blood vessel of a tubular prosthetic graft and its delivery via a catheter to the desired location within the vascular system. The alternative approach to percutaneous revascularization is the surgical placement of vein, artery, or other by-pass segments from the aorta onto the coronary artery, requiring open heart surgery, and significant morbidity and mortality. Advantages of the percutaneous revascularization method over conventional vascular surgery include obviating the need for surgically exposing, incising, removing, replacing, or by-passing the defective blood vessel, including heart--lung by-pass, opening the chest, and general anesthesia.
Revascularization via a prosthetic graft device is desirable in various situations to expand a constricted vessel or to maintain an open passageway through a vessel. A stent may be used to expand a vascular channel or to maintain the expanded lumen, after angioplasty of a coronary artery. In these situations, stents are useful to prevent restenosis of the dilated vessel, to prevent elastic recoil of the vessel, or to eliminate the danger of occlusion caused by "flaps" resulting from intimal tears associated with angioplasty. Stents may be utilized after atherectomy which is the cutting out of plaque to remove it. In such removal of atherosclerotic plaque from the coronary vessel wall, a stent is used to maintain patency of the vessel. Stents are used in by-pass grafts as well, to maintain patency. Stents can also be used to reinforce collapsing structures in the respiratory, biliary, urological, and other tracts.
Existing technology for percutaneous treatment of coronary artery disease either uses balloons or other devices to break open coronary plaque. An angioplasty balloon is inflated within the stenosed vessel, or body passageway, in order to shear and disrupt the wall components of the vessel to obtain an enlarged lumen. This procedure leaves an irregular surface and exposes thrombogenic areas. Clot occurs in some patients, occasionally causing abrupt closure of the coronary artery or vein by-pass segment; in a major portion of angioplastied sites, plaque recurs (restenosis). Thus, although the body passageway may initially be successfully expanded by a balloon dilation procedure, subsequent, early restenosis can occur due to the recoil of the body passageway wall as well as intimal growth which decreases the size of the previously expanded lumen of the body passageway.
Stents, which, prop open vessels, preventing recoil and treating dissections caused by angioplasty have been recently approved for clinical use in the United States. Structures which have previously been used as stents or intraluminal vascular grafts have included coil stainless steel springs; helically wound coil springs manufactured from an expandable heat-sensitive material; and expanding stainless steel stents formed of stainless steel wire in a zigzag or other pattern. These stents are highly thrombogenic and require vigorous anticoagulation, with significant, and occasionally life threatening side effects. Important obstacles encountered after stent placement include coagulation (clot) formation on surfaces of the stent, inflammation around the stent, and undue proliferation of neointima. Because these problems persist, as described in the New England Journal of Medicine (Vol. 331, pages 489-501 and 539-541, 1994), procedures are performed using post-stent treatments, such as anticoagulation drugs. Such drugs are costly and their use may lead to complications, such as uncontrolled hemorrhage, bleeding, and vascular complications. A number of attempts have been made to make stents more compatible with the patient's coagulation system. Foreign materials whether bare metal, polymers, or other materials attract thrombin and platelets as well as other blood constituents that promote the problems described immediately above. Attempts have been made to alter the surface charge, to apply a polymer coating on the stent and to impregnate polymer coatings with drugs such as heparin to enhance tissue and body acceptability, but these approaches themselves have certain deficiencies. Although vascular stents are used in humans, and for experimental purposes in animals, it is desirable to overcome their deficiencies.
Therefore, what is needed is a new design for graft prosthesis, a method for preparing it, and a method for inserting it which avoids complications due to coagulation, restenosis, recoil, inflammation, and other problems.